374 THE WORLD OF THE SEA. 
When the /'w/mar, or stormy petrel, is seized, it vomits an oil 
the colour of amber, which is highly valued by the inhabitants 
of St. Kilda as a good external remedy in many disorders, 
particularly rheumatism. This oil is also burnt, the best being 
procured from old birds. They catch the fulmars at night, 
THE BLACK DUCK. 
(Anas nigra.) 
and press their beaks, when each bird yields about two spoonfuls 
of oil. 
The valuable material known as gzano, or, properly speaking, 
huanu, so prized by agriculturalists, is a product of aquatic birds. 
Guano is a mass of excrements deposited far out at sea on 
rocks and islets. It is calculated that a bird of average size 
furnishes about one ounce a day. Now, on certain rocks, beds of 
guano are formed as much as 100 feet deep; it must have taken 
hundreds of years and millions of birds to form these deposits. 
The Island of Cincha, near Peru, 100 miles south of Callao, is 
